This study is designed to provide information concerning morphologic changes in the intestinal absorptive cell in particular and in the intestinal mucosa in general during the process of normal fat absorption and in diseases that lead to intestinal malabsorption. Experiments are as follows. 1) To elucidate the ultrastructural basis, if any, for intestinal malabsorption as seen in patients with Addison's disease, agammaglobulinemia, apparently primary dermatologic disorders, thyrotoxicosis, ulcerative colitis, iron deficiency, diabetics with severe malabsorption not explained by pancreatic insufficiency or by changes in intestinal biopsy as seen at light microscopy, and in drug induced steatorrhea. 2) To elucidate the morphologic route by which macromolecules of the size of albumin are transported across capillary and lymphatic endothelia of the intestinal mucosa. 3) To elucidate the morphology of short chain and medium chain fatty acid absorption. 4) To demonstrate, if possible, using radioautographic methods, the site at which the protein component of chylomicrons is synthesized and added to the chylomicron. The objective of these studies is to provide us with knowledge in depth as to the ultrastructural and functional basis of intestinal absorption and malabsorption of fats. The related study of the mechanism of transport of macromolecules by intestinal mucosal capillaries and lymphatics should also aid us in our understanding of the role of the intestinal mucosa in absorption in general and in the protein losing enteropathies in particular.